Bifunctional electrolyte additive sustains high efficient zinc-iodine batteries via respirable-interphase formation and polyiodide ion suppression

Aqueous zinc–iodine batteries hold immense potential for grid-scale storage owing to their inherent safety, cost-effectiveness, and eco-friendliness. However, their commercialization is hindered by limited cycle life caused by irreversible Zn plating/stripping and I₂/I⁻ redox reaction. Herein, we propose 1-(2-hydroxyethyl)-1-methylguanidine dihydrogen phosphate (HMDP) as an effective bifunctional additive that not only stabilizes Zn anode by forming a respirable-interphase but also suppresses the generation of polyiodide ion at the cathode by separating I₂ and I⁻. The Zn//Cu cells with HMDP achieve a Coulombic efficiency of 99.63 % over 650 cycles (2 mA cm⁻², 1 mAh cm⁻²) and retain 85.83 % initial efficiency after 7-day calendar aging. Surprisingly, the Zn//Zn symmetric cells with HMDP deliver an ultra-long cycle-life of 5150 h (0.5 mA cm⁻², 0.5 mAh cm⁻²) thanks to the formation of respirable-interphase. Meanwhile, the Zn//I₂ full cells retain 81.04 % capacity at 2 A g⁻¹ after >3000 cycles due to the suppression of polyiodide ion. This work establishes strategies of I₂–I⁻ separator and respirable-interphase to design advanced zinc-iodine batteries.